Patent application title: MODEL REFERENCE IDENTIFICATION AND CANCELLATION OF MAGNETICALLY-INDUCED VOLTAGES IN A GRADIENT MAGNETIC FIELD

Abstract:

Systems and methods of dynamically controlling an implanted medical device
located within a patient's body in the presence of a gradient magnetic
field or other external interference are disclosed. The system can
include a reference model of the implanted medical device and of body
tissue within the patient's body in the absence of a gradient magnetic
field, and a control unit configured to dynamically control voltages or
currents applied to a lead of the implanted medical device based on
predicted parameters determined by the reference model.

Claims:

1. A method of dynamically controlling an implanted medical device located
within a patient's body in the presence of an external interference, the
method comprising:creating a model of an implantable lead and of body
tissue within the body;detecting the presence of an external interference
within the body;measuring the response of an excitation voltage or
current applied to the lead in the presence of the external
interference;comparing the measured response obtained in the presence of
the external interference with a predicted response outputted by the
model; anddynamically modifying a voltage or current applied to the lead
based at least in part on the predicted response outputted by the model.

2. The method of claim 1, wherein creating a model of an implantable lead
and of body tissue within the body includes:measuring the response of an
excitation voltage or current applied to the lead in the absence of an
external interference;comparing the measured response in the absence of
the external interference to a predicted response generated by the model
and outputting an error signal; andadjusting one or more model parameters
of the model to minimize the error signal.

3. The method of claim 1, wherein the implantable lead is a bipolar lead
including a plurality of lead electrodes electrically coupled to a pulse
generator, and wherein creating a model of an implantable lead and of
body tissue within the body includes creating a model of the lead
implanted within the heart and of the cardiac tissue disposed between the
lead electrodes.

4. The method of claim 3, wherein the model includes one or more impedance
parameters associated with the implanted lead and the cardiac tissue
between the lead electrodes.

5. The method of claim 1, wherein the implantable lead is a unipolar lead
including an electrode electrically coupled to a pulse generator, and
wherein creating a model of an implanted lead and of body tissue within
the body includes creating a model of the lead implanted within the heart
and of the body tissue disposed between the electrode and the pulse
generator.

6. The method of claim 5, wherein the model includes one or more impedance
parameters associated with the implanted lead and the body tissue between
the electrode and the pulse generator.

7. The method of claim 1, wherein creating a model of an implantable lead
and of body tissue within the body is performed in the absence of the
external interference.

8. The method of claim 1, wherein measuring the response of the excitation
voltage or current includes measuring the current in the implanted lead.

9. The method of claim 1, wherein dynamically modifying the voltage or
current applied to the lead based on the predicted response outputted by
the model includes minimizing an error signal generated by comparing the
measured response of the lead in the presence of the external
interference to a predicted response from the model determined in the
absence of the external interference.

10. The method of claim 9, wherein dynamically modifying the voltage or
current applied to the lead based on the predicted response outputted by
the model is performed by a control unit including a closed-loop feedback
controller.

11. A method of dynamically controlling an implanted medical device
located within a patient's body in the presence of a gradient magnetic
field, the method comprising:measuring the response of an excitation
voltage or current applied to a lead implanted in or near the heart in
the absence of a gradient magnetic field;creating a model of the
implanted lead and of body tissue within the body, the model including
one or more impedance parameters associated with the implanted lead and
the body tissue;comparing the measured response in the absence of the
gradient magnetic field to an anticipated response generated by the model
and outputting an error signal;adjusting one or more of the model
parameters to minimize the error signal;detecting the presence of a
gradient magnetic field within the body;measuring the response of an
excitation voltage or current applied to the lead in the presence of the
gradient magnetic field;comparing the measured response obtained in the
presence of the gradient magnetic field with a predicted response
outputted by the model; andmodifying a voltage or current applied to the
lead based at least in part on the predicted response from the model.

12. The method of claim 11, wherein the lead is a bipolar lead including a
plurality of lead electrodes electrically coupled to a pulse generator,
and wherein creating a model of the implanted lead and of body tissue
includes creating a model of the lead implanted within the heart and of
the cardiac tissue disposed between the lead electrodes.

13. The method of claim 12, wherein the one or more impedance parameters
includes an impedance parameter associated with the implanted lead and an
impedance parameter associated with the cardiac tissue between the lead
electrodes.

14. The method of claim 11, wherein the lead is a unipolar lead including
an electrode electrically coupled to a pulse generator, and wherein
creating a model of the implanted lead and of body tissue includes
creating a model of the lead implanted within the heart and of the body
tissue disposed between the electrode and the pulse generator.

15. The method of claim 14, wherein the one or more impedance parameters
includes an impedance parameter associated with the implanted lead and an
impedance parameter associated with the body tissue between the electrode
and the pulse generator.

16. The method of claim 11, wherein creating a model of the lead is
performed in the absence of the gradient magnetic field.

17. The method of claim 11, wherein measuring the response of the
excitation voltage or current includes measuring the current in the
implanted lead.

18. The method of claim 11, wherein dynamically modifying the voltage or
current applied to the lead based on the predicted response outputted by
the model includes minimizing an error signal generated by comparing the
measured response of the lead in the presence of the magnetic field to a
predicted response outputted from the model.

19. The method of claim 18, wherein dynamically modifying the voltage or
current applied to the lead based on the predicted response outputted by
the model is performed by a control unit including a closed-loop feedback
controller.

20. A system for cancelling magnetically-induced voltages on an implanted
medical device having a lead implanted in or near the heart, the system
comprising:a reference model of the lead including one or more model
parameters associated with the lead and the heart; anda control unit
adapted to control a voltage or current applied to the lead in the
presence of a gradient magnetic field;wherein the control unit is
configured to dynamically control the voltage or current based at least
in part on the one or more model parameters of the model.

21. The system of claim 20, further comprising a sensor configured to
sense the presence of a magnetic field within the body.

22. The system of claim 20, wherein the model parameters include an
impedance parameter associated with the lead and an impedance parameter
associated with the heart.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001]This application claims priority under 35 U.S.C. §119 to U.S.
Provisional Application No. 61/029,743, filed on Feb. 19, 2008, entitled
"Model Reference Identification and Cancellation Of Magnetically-induced
Voltages In A Gradient Magnetic Field," which is incorporated herein by
reference in its entirety.

TECHNICAL FIELD

[0002]The present invention relates generally to implantable medical
devices and the delivery of diagnostic and therapeutic treatments during
medical procedures such as magnetic resonance imaging (MRI). More
specifically, the present invention relates to the identification and
cancellation of magnetically-induced voltages in a gradient magnetic
field or other external noise.

BACKGROUND

[0003]Magnetic resonance imaging (MRI) is a non-invasive imaging method
that utilizes nuclear magnetic resonance techniques to render images
within a patient's body. Typically, MRI systems employ the use of a
static magnetic coil having a magnetic field strength of between about
0.2 to 3 Teslas. During the procedure, the body tissue is briefly exposed
to RF pulses of electromagnetic energy in a plane perpendicular to the
magnetic field. The resultant electromagnetic energy from these pulses
can be used to image the body tissue by measuring the relaxation
properties of the excited atomic nuclei in the tissue. Pulsed gradient
magnetic fields are used for spatial variation of static fields for image
phase, frequency encoding, and slice selection within the body.

[0004]During imaging, the electromagnetic radiation produced by the MRI
system may be picked up by implantable device leads used in implantable
medical devices such as pacemakers or cardiac defibrillators. This energy
may be transferred through the lead to the electrode in contact with the
tissue, which may lead to elevated temperatures at the point of contact.
The degree of tissue heating is typically related to factors such as the
length of the lead, the conductivity or impedance of the lead, and the
surface area of the lead electrodes. Exposure to a magnetic field such as
a pulsed gradient magnetic field may also induce an undesired voltage in
the lead.

SUMMARY

[0005]The present invention relates generally to the identification and
cancellation of voltages induced on an implanted medical device located
within a patient's body in the presence of a gradient magnetic field or
other external noise. An illustrative system for identifying and
cancelling magnetically-induced voltages on an implanted medical device
having a lead implanted in or near the heart includes a reference model
including one or more impedance parameters associated with the lead and
of the body tissue within the patient's body, and a control unit adapted
to control a voltage or current applied to the lead in the presence of a
magnetic field. In some embodiments, the control unit is configured to
dynamically control the voltage or current applied to the lead based on
the one or more impedance parameters stored within the model. In use, and
in some embodiments, the control unit is configured predict the variables
(e.g., voltage or current) to be applied to the lead in the presence of
the magnetic field in order to compensate for the effects of the field.

[0006]An illustrative method of dynamically controlling an implanted
medical device located within a patient's body in the presence of a
gradient magnetic field includes creating a model of the implanted lead
and of body tissue within the body in the absence of a magnetic field,
detecting the presence of a gradient magnetic field within the body,
measuring the response of an excitation voltage or current applied to the
lead in the presence of the gradient magnetic field, comparing the
measured response against a modeled response obtained by the model in the
absence of a gradient magnetic field, calculating the error between the
measured response and the desired, modeled response, and modifying the
voltage or current applied to the lead based on the calculated error to
steer the measured response towards the desired response.

[0007]While multiple embodiments are disclosed, still other embodiments of
the present invention will become apparent to those skilled in the art
from the following detailed description, which shows and describes
illustrative embodiments of the invention. Accordingly, the drawings and
detailed description are to be regarded as illustrative in nature and not
restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a schematic view of an illustrative medical device having
a lead implanted within the body of a patient and subjected to a gradient
magnetic field;

[0009]FIG. 2 is a block diagram showing several illustrative components of
the pulse generator of FIG. 1;

[0010]FIG. 3 is a block diagram showing an illustrative model-based system
for modeling a medical device implanted within the body;

[0011]FIG. 4 is a block diagram showing the model-based system of FIG. 3
in the presence of a gradient magnetic field; and

[0012]FIG. 5 is a flow chart showing an illustrative method of dynamically
controlling an implanted medical device in the presence of a gradient
magnetic field.

[0013]While the invention is amenable to various modifications and
alternative forms, specific embodiments have been shown by way of example
in the drawings and are described in detail below. The intention,
however, is not to limit the invention to the particular embodiments
described. On the contrary, the invention is intended to cover all
modifications, equivalents, and alternatives falling within the scope of
the invention as defined by the appended claims.

DETAILED DESCRIPTION

[0014]FIG. 1 is a schematic view of an illustrative medical device 12
equipped with a lead implanted within the body of a patient. In the
illustrative embodiment depicted, the medical device 12 includes a pulse
generator 14 implanted within the patient's body and a lead 16 (e.g., a
unipolar or bipolar lead) placed at a location in or near the patient's
heart 18. The heart 18 includes a right atrium 20, a right ventricle 22,
a left atrium 24, and a left ventricle 26. The pulse generator 14 can be
implanted subcutaneously within the body, typically at a location such as
in the patient's chest or abdomen, although other implantation locations
are possible.

[0015]A proximal portion 28 of the lead 16 can be coupled to or formed
integrally with the pulse generator 14. A distal tip portion 30 of the
lead 16, in turn, can be implanted at a desired location in or near the
heart 18 such as the right ventricle 22, as shown. Although the
illustrative embodiment depicts only a single lead 16 inserted into the
patient's heart 18, in other embodiments multiple leads can be utilized
so as to electrically stimulate other areas of the heart 18. In some
embodiments, for example, the distal portion of a second lead (not shown)
may be implanted in the right atrium 20. In addition, or in lieu, another
lead may be implanted in or near the left side of the heart 18 (e.g., in
the coronary veins) to stimulate the left side of the heart 18. Other
types of leads such as epicardial leads may also be utilized in addition
to, or in lieu of, the lead 16 depicted in FIG. 1.

[0016]During operation, the lead 16 can be configured to convey electrical
signals from the pulse generator 14 to the heart 18. For example, in
those embodiments where the pulse generator 14 is a pacemaker, the lead
16 can be used to deliver electrical therapeutic stimulus for pacing the
heart 18. In those embodiments where the pulse generator 14 is an
implantable cardiac defibrillator, the lead 16 can be utilized to deliver
electric shocks to the heart 18 in response to an event such as a heart
attack or ventricular tachycardia. In some embodiments, the pulse
generator 14 includes both pacing and defibrillation capabilities.

[0017]When the pulse generator 14 is subjected to a gradient magnetic
field, as shown generally by arrow "B" in FIG. 1, a magnetically-induced
voltage (VEMF) is delivered to the lead 16 that interferes with the
therapeutic electrical signals delivered by the lead 16. During an MRI
procedure, for example, a rapidly changing magnetic field B produced by
an energized MRI coil induces an electromotive force voltage on the lead
16 that combines with the excitation voltage normally generated by the
pulse generator 14. The voltage VEMF produced can be determined from
Faraday's Law as follows:

V EMF = B t A ##EQU00001##

Thus, as can be understood from the above equation, the magnitude of the
induced voltage VEMF is dependent on the time rate of change of the
magnetic field

B t ##EQU00002##

and the effective area A upon which the gradient magnetic field acts.

[0018]For a unipolar lead 16 such as that depicted in FIG. 1, the
effective area A upon which the gradient magnetic field B acts is defined
generally as the area that is bounded by the length of the lead 16 from
the proximal end 28 to the distal tip 30 and the distance from the lead
tip 30 to the pulse generator 14. For a bipolar lead configuration, the
effective area A upon which the gradient magnetic field B acts is
typically less than for a unipolar lead, and is defined generally by the
area between the tips of the lead electrodes. In either configuration,
the voltage VEMF appears as a voltage source in the circuit loop 32
formed by the pulse generator 14, the lead 16, and the tissue impedance
between the pulse generator 14 and the lead tip 30. This voltage
VEMF induces a current on the lead 16 along with the desired
therapeutic stimulus current generated by the pulse generator 14. During
operation, this voltage VEMF can result in inappropriate currents on
the lead 16 that are then transmitted into the surrounding cardiac
tissue. The induced voltage VEMF may also result in device
inhibition or inappropriate detections leading to charge timeout faults.

[0019]FIG. 2 is a block diagram showing several illustrative components of
the pulse generator 14 of FIG. 1. As shown in FIG. 2, the pulse generator
14 includes a control unit 34 adapted to run a control algorithm 36,
which as discussed further herein, can be used to provide closed-loop
control over the excitation voltage or current applied to the pulse
generator lead 16 in the presence of a gradient magnetic field or other
external noise. The control unit 34 can be coupled to other components of
the pulse generator 14, including a timer circuit 38 for taking time and
date measurements, an energy source 40 such as a rechargeable battery or
power capacitor, and a storage memory 42 such as a flash memory or
ferroelectric memory for storing data and commands used by the pulse
generator 14.

[0020]The control unit 34 can further include control circuitry for
controlling various other implantable medical devices coupled to the
pulse generator 14, including one or more remote sensing devices 44
and/or therapy delivery devices 46. Examples of remote sensing 44 devices
that can be coupled to the pulse generator 14 can include, but are not
limited to, pressure sensors, accelerometers, pulmonary sound sensors,
chemical sensors, and temperature sensors. In one embodiment, for
example, the control unit 34 can be coupled to a magnetic sensor such as
a reed switch or Hall-effect sensor that can be used to detect the
presence of magnetic fields within the body.

[0021]In the illustrative embodiment of FIG. 2, the control algorithm 36
includes a lead and tissue reference model 48 used by the algorithm 36 to
model one or more physical characteristics of the pulse generator lead 16
and the cardiac tissue adjacent to the lead 16. In some embodiments, for
example, the reference model 48 includes an impedance parameter
associated with the lead 16 as well as an impedance parameter associated
with the cardiac tissue between the lead tip 30 and the pulse generator
14. In certain embodiments, the model parameters are stored as parameters
within a look-up table or the like, and can be used by the control
algorithm 36 to control the operation of the pulse generator 14,
including the timing and magnitude of excitation voltage or current
signals applied to the lead 16.

[0022]FIG. 3 is a block diagram showing an illustrative model-based system
50 for modeling an implanted medical device such as the pulse generator
of FIG. 2. As shown in FIG. 3, an excitation voltage V0 generated by
the pulse generator 14 is applied to block 52, which represents the
actual load characteristics of the lead 16 and the cardiac tissue. In the
illustrative embodiment of FIG. 1, for example, the lead and tissue block
52 may represent the impedance of the lead 16 and the impedance of the
cardiac tissue between the pulse generator 14 and the lead tip 30. In
those embodiments in which a bipolar lead is used, the lead and tissue
block 52 may represent the impedance of the cardiac tissue between the
exposed portions of the electrodes.

[0023]In the absence of a gradient magnetic field, an accurate reference
model 48 of the lead and cardiac tissue is developed based on one or more
measured responses 54 from the lead and tissue 52. In some embodiments,
for example, the reference model 48 may be generated by comparing via a
comparator 58 a measured electrical current 54 from the implanted lead
and body tissue 52 against a predicted electrical current 56 generated by
the model 48 in the absence of a magnetic field. Measurement of the
electrical current 54 within the lead 16 can be taken, for example, by a
current sensor or by sensing a voltage drop across a reference resistor
in series with the lead 16 and the body tissue.

[0024]Based on the responses 54,56 received from both the measured (i.e.,
actual) parameters and the model-based parameters, the comparator 58
outputs an error signal 60 indicating the error in the model's predicted
response. This error signal 60 is then fed to the control unit 34, which
updates one or more parameters in the model 48 to better simulate the
actual lead and tissue impedance 52, thus reducing the model error. If,
for example, the model 48 approximates a lead and tissue impedance that
is greater than the actual, measured impedance, the control unit 34 may
decrease the modeled impedance within the model 48 to minimize the error,
similar to that performed by a closed-loop feedback controller such as a
proportional-integral-derivative (PID) controller. Other suitable model
parameter identification techniques including, but not limited to, least
squares estimation (LS), maximum likelihood estimation (MLE), and best
linear unbiased estimates (BLUE) may also be used to accurately calculate
the model parameters.

[0025]The process of updating the model 48 to better approximate the
actual lead and body tissue characteristics can be performed
continuously, at predetermined time periods, or in response to a control
signal from another device. In some embodiments, for example, the process
of updating the model 48 can be accomplished initially when the lead is
implanted within the body, and then subsequently at predetermined time
intervals (e.g., every five minutes) until a static magnetic field is
detected within the body, indicating the presence of a gradient magnetic
field. In certain embodiments, model parameters such as the impedance of
the lead 16 and the surrounding cardiac tissue, for example, can be
updated by the control unit 34 by continuously or periodically measuring
the voltage across a reference resistor coupled to the lead 16, and then
comparing the measured voltage against a predicted current outputted by
the model 48.

[0026]FIG. 4 is a block diagram showing the model-based system 50 of FIG.
3 in the presence of a gradient magnetic field. In some embodiments, the
presence of a static magnetic field within the body during an MRI
procedure can be detected by a magnetic sensor such as a Hall effect
sensor, reed switch, or the like. From this sensed static magnetic field,
the control unit 34 may then deduce that a gradient magnetic field is
present within the body, activating an MRI mode within the control unit
34 to compensate for the induced EMF voltage VEMF at the pulse
generator 14. Other techniques can also be used to determine the presence
of the gradient magnetic field to activate the MRI mode within the
control unit 34. In some embodiments, for example, a signal received from
an external device in communication within the pulse generator 14 (e.g.,
from the MRI device) may signal the presence of the gradient magnetic
field, causing the control unit 34 to enter into the MRI mode of
operation.

[0027]In the presence of a gradient magnetic field, a magnetically-induced
voltage VEMF produces a current that is transmitted through the lead
and body tissue 52, which affects the measured response 54. In some
cases, for example, the presence of the voltage VEMF may result in a
greater amount of current within the lead than is desired for performing
the therapy. This increase in response 54 is then compared against the
response 56 predicted by the model 48 in the absence of the magnetic
field to produce an error signal 60 that is fed to the control unit 34.
In those embodiments where the actual and modeled responses 54,56 are
currents, the error signal 60 may represent, for example, the magnitude
of the difference between the actual current outputted by the lead and
the predicted current outputted by the model 48.

[0028]Based on the error signal 60, the control unit 34 seeks to minimize
the error between the actual lead and body tissue response 54 and the
predicted (i.e., desired) response 56 generated by the model 48 in the
absence of the magnetic field. In some embodiments, the control unit 34
then feeds a compensation voltage signal 62 to an adder 64, which is then
added to the excitation voltage V0 applied to the leads as voltage
signal 66. In this manner, the control unit 34 dynamically modifies the
voltage signal 66 applied to the lead and body tissue 52 so that the
actual response 54 from the system 50 is similar to the modeled response
56 in the absence of the magnetic field. The output from the system 50
can be continuously compared with the measured responses 54 from the
actual lead and body tissue 52, allowing the control unit 34 to maintain
the error minimization between the model 48 and the lead and body tissue
52.

[0029]FIG. 5 is a flow chart showing an illustrative method 68 of
dynamically controlling an implanted medical device in the presence of an
external interference such as a gradient magnetic field. The method 68
begins generally at block 70 with the step of measuring the response of
an excitation voltage or current applied to a lead implanted at a
location within the body prior to the presence of a gradient magnetic
field. In some embodiments, for example, measuring the response of the
excitation voltage applied to the lead can include measuring the
electrical current in the lead as a result of an excitation voltage
generated by the pulse generator. Measurement of the electrical current
within the lead can be accomplished, for example, by sensing the voltage
across a reference resistor coupled to the lead, using a bridge circuit
to sense the current within the lead, and/or by other suitable technique.
From these measurements, the impedance characteristics of the implanted
lead and body tissue are then determined.

[0030]From the measured response to the excitation voltage, one or more
characteristics of the lead and body tissue are then analyzed to create a
model of the lead and body tissue (block 72). In some embodiments, for
example, the impedance characteristics of the lead and the body tissue
are modeled based on one or more measured responses to an excitation
voltage applied to the lead. The model may include, for example, an
impedance parameter associated with the lead and an impedance parameter
associated with the patient's heart.

[0031]Once a model is created representing the impedance of the implanted
lead and body tissue, an anticipated response from the model resulting
from the applied excitation voltage can then be compared against the
actual, measured response produced by the excitation voltage to generate
an error signal (block 74). The error signal can then be used by the
control unit to adjust the model parameters to better approximate the
actual, measured response from the implanted lead and body tissue in
order to minimize the error signal (block 76). In some embodiments, for
example, the model parameters can be continually or periodically adjusted
by the pulse generator to minimize the error produced by the model, or to
compensate for any changes to the lead and tissue impedance that occur
over time.

[0032]The pulse generator can be configured to detect the presence of a
magnetic field within the body (block 78), and then initiate a control
algorithm within the control unit that dynamically modifies the voltage
applied to the lead based on the parameters stored within the model
(block 80). In some embodiments, for example, the pulse generator
includes a sensor such as a Hall-effect sensor or reed-switch that can
detect a static magnetic field or other external noise within the body
produced by an MRI device, indicating the presence of a gradient magnetic
field. The signal from the sensor can then be fed to the control unit,
prompting the control unit to modify the voltage applied to the lead
based on the model. In other embodiments, the pulse generator can be put
into an MRI mode via a signal received from an external device (e.g., the
MRI device), from another external device, or from another device
implanted within the body.

[0033]Upon the detection of the magnetic field, the control unit can be
configured to compare the actual response of the system in the presence
of the magnetic field with the predicted response outputted by the model
and determined in the absence of the magnetic field (block 82). Based on
this comparison, an error signal is created which is then fed to the
control unit for use in modifying the excitation voltage or current
applied to the lead. In some embodiments, for example, the error signal
may represent the magnitude of the difference between the current that is
presently being applied to the lead and the current predicted by the
model in the absence of the magnetic field. Based on the error signal,
the control unit then seeks to minimize the error signal by dynamically
modifying the voltage or current applied to the lead (block 84).

[0034]Various modifications and additions can be made to the exemplary
embodiments discussed without departing from the scope of the present
invention. For example, while the embodiments described above refer to
particular features, the scope of this invention also includes
embodiments having different combinations of features and embodiments
that do not include all of the described features. Accordingly, the scope
of the present invention is intended to embrace all such alternatives,
modifications, and variations as fall within the scope of the claims,
together with all equivalents thereof.